Fring is a cool iPhone application that allows you to use the mobile community at Fring to talk, chat & interact with other fring community members. That includes using Fring, Skype, MSN, gtalk, ICQ, Twitter, Yahoo!, and AIM on your iPhone. You can also use 3G, WiFi, GPRS, and Edge to stay connected from anywhere in the world.
Not only you can instant message all day with Fring, you make VOIP calls over Wi-Fi and save money, and the application does have a dynamic contact list that makes it easy to stay connected (keep in mind that AT&T has some restrictions with VOIP usage on the iPhone).
Fring is free and definitely worth a try.